The present invention relates to novel zinc-based alloys as well as to the preparation and use thereof for producing thermal-sprayed coatings having improved corrosion resistance and adherence.
Thermal-spraying is a generic term designating a type of method according to which molten or semi-molten particles are propelled and allowed to strike a surface in a uniform manner to form a coating. Examples of such methods include flame-spraying and plasma-spraying as well as the so-called detonation gun process and jet coat process, which are all well known in the art.
Thermal spraying allows the production of coatings of a wide variety of materials provided that the coating material does not sublimate, decompose or excessively vaporize during thermal spraying. Metals, alloys, ceramics and polymers can thus be sprayed on almost any substrates such metals, plastics, wood, ceramics and composites. Thermal-sprayed coatings are used in many industrial applications to protect parts against degradation such as that caused by corrosion in a gas or liquid at ambient or elevated temperature, or wear by a gas, liquid or solid in an aggressive environment at ambient or elevated temperature. Thermal-sprayed coatings are also used for producing unique operating mechanical systems such as thermal barrier coatings or clearance control abradable seals for jet-engines, for reclamation of worn parts by spraying material where volume losses have occurred, for lubrication at high temperature and for producing various coatings having special purposes in the electronic, printing, drilling, atomic, aeronautic, mining and chemical industries.
Thermal-sprayed coatings can comprise only one layer of material or a plurality of layers of different materials. In the case of multi-layered coatings, the layer on the substrate is generally designated as a bond coat since most of the time its function is to serve as anchorage for other types of material; on the other hand, the last layer to be deposited is generally referred to as top coat. Bond coats have been developed to significantly increase performance and reliability of coating systems. On an historical basis, the development of bond coat materials have evolved from molybdenum in the early 1940's, to nickel-chromium alloys in the 1950's, to nickel-aluminum composites in the 1960's, to aluminium bronze in the 1970's, and to pre-alloyed nickel aluminium. All these bond coat materials have been primarily developed to increase the adherence of coatings and in some cases to provide at the same time a good oxidation resistance, and they are thus not suitable for protecting parts against aqueous corrosion in humid environment as found in outdoor structures. In this later case, coatings based on zinc, aluminium or their alloys have been particularly studied and have been extensively utilized. Thermal-sprayed aluminium coatings have been developed for U.S. Navy ships for corrosion control. These aluminum-based coatings present important drawbacks since they ave a residual porosity which is detrimental. Very effective organic sealer must be used to impede the penetration of water when such aluminium-based coatings are used. Moreover, these coatings cannot be used as a bond coat due to the presence of an organic sealer. Thermal-sprayed coatings have also been used for protection of outdoor structures in a wide range of environment. Zinc and zinc-aluminum alloys have been particularly successful in protecting large structures such as bridges in many countries. In this case, the coating is only used for aesthetic and corrosion control purposes. The adherence of these coatings is relatively low and they are thus unsuitable for use as bond coat.